1. Field of the Invention:
The present invention relates to a transmissive filter for correcting illuminance irregularities in an image forming area in an image forming optical system.
2. Description of the Prior Art:
It is known in image forming optical systems such as electrophotographic copying apparatus for copying original image films, facsimile transmitter/receivers, and the like that if an original image, an object, or the like is uniformly illuminated with light, the brightness on a surface on which an image of the illuminated object is formed is progressively lowered in a direction away from the optical axis resulting in illuminance irregularities (exposure irregularities) due to the cosine law.
It has been customary to correct such illuminance irregularities on the image forming surface by placing, between the image forming surface and a focusing lens for focusing an image on the image forming surface, or between the original image and the focusing lens, either a transmissive filter composed of a transparent base plate having thereon opaque dots which are closely spaced in a region near the optical axis and becomes progressively coarser toward the peripheral edge of the filter, or an area-type filter having an attenuating region near the optical axis.
These conventional filters have their transmittance progressively increased from the optical axis toward the peripheral edge. With this arrangement of known nature, however, the illuminance error developed upon illuminance correction is considerably large dependent on the height of the object, as shown in FIG. 31 of the accompanying drawings. Moreover, the prior filters are not sufficiently effective in eliminating illuminance unevenness which is caused by changes in the field angle of the focusing lens at the time of enlarging or reducing the size of the image.